1. Field of the Invention
This invention relates to a solid golf ball comprising a solid core and a two-layer cover it relates to particularly, such a golf ball which is improved in spin receptivity, feel and control at no sacrifice of flight performance and durability.
2. Prior Art
Golf balls having a variety of constructions are available today on the market. The majority of commercial golf balls are either two-piece solid golf balls comprising a rubber-based core and a cover composed of ionomer resin or the like, or thread-wound golf balls comprising a thread-wound core obtained by winding rubber thread about a solid or liquid center, and a cover: formed over the core.
Most golfers of ordinaryl skill use two-piece solid golf balls because of their excellent flight performance and durability. However, these balls have a very hard feel and are poor in control due to the rapid separation of the ball from the club head. For this reason, many professional golfers and skilled amateurs prefer using thread-wound balls to two-piece solid balls. Yet, although thread-wound golf balls have a superior feel and control, their distance and durability fall short of those for two-piece balls.
Thus, two-piece solid golf balls and thread-wound golf balls today provide mutually opposing features, and so golfers select which type of b,all to use based on their level of skill and personal preference.
In order to produce solid golf balls presenting a feel close to that of wound golf balls when hit, the inventors made a number of proposals about three-piece solid golf balls comprising a solid core, a cover inner layer and a cover outer layer, as disclosed in JP-B 8301/1995, JP-A 24084/1995 and JP-A 10358/1997. These patents disclose golf balls wherein the cover outer layer is formed harder than the cover inner layer, and the balls are designed to acquire a lower spin rate upon shots intended for distance like driver shots.
These galls are advantageous in flight distance, but due to the low spin structure, they are difficult to stop when the player desires to stop the ball at the destination on an iron shot.
By contrast, many proposals were also made on golf balls which are improved in control, as disclosed in JP-B 4110/1993 and JP-A 24085/1995. These golf balls are characterized in that the cover inner layer is formed harder than the cover outer layer. It is intended to increase spin by softening the cover outer layer to increase the friction of the ball with the club face. Despite the advantage of increased spin, these golf balls, however, sometimes fail to increase the flight distance because of the soft cover outer layer inviting a loss of resilience.
Therefore, an object of the invention is to provide a solid golf ball which has spin receptivity close to wound golf balls, that is, receives an appropriate spin on iron shots, and is improved in control and feel, at no sacrifice of the superior flight performance and durability inherent to solid golf balls.
The invention is directed to a solid golf ball comprising a solid core formed of a base rubber and a cover of two-layer structure consisting of an inner layer surrounding the solid core and an outer layer surrounding the inner layer. According to the invention, the cover inner layer is composed mainly of a thermoplastic elastomer free of an ionomer resin, and the cover outer layer is formed of a cover stock containing 50 to 100% by weight of an ionomer resin. These cover layers are adjusted such that the difference in Shore D hardness between the inner layer and the outer layer is within 2 Shore units. Then, while maintaining the superior flight performance and durability inherent to solid golf balls, the ball is improved in feel and spin receptivity. The spin receptivity of the ball is increased to a level close to that of wound golf balls. Then the ball will receive an appropriate spin on iron shots. The ball thus becomes easier to control and pleasant to the feel.
In one preferred embodiment, the cover inner layer is formed such that the ratio of M400%/M100% is up to 1.9/1, wherein M100% is the strength :of the inner layer at a tensile elongation of 100% and M400% is the strength of the inner layer at a tensile elongation of 400%. Upon impacts with weaker forces as on approach shots, the ball undergoes more deformation and hence, receives more spin.
In further preferred embodiments, the cover inner layer and the cover outer layer each have a Shore D hardness in the range of 40 to 60; and the solid core is based on polybutadiene and undergoes a deflection of 2.5 to 5.5 mm under an applied load of 100 kg. Then the aforementioned advantages are enhanced.